1. Field of the Invention
The present invention relates to a magneto-optical recording medium for recording and erasing information thereon/therefrom by utilizing a rise in temperature upon the irradiation of laser beams and for reading out a recorded signal by utilizing magneto-optical effects, and also relates to a readout method of the same.
2. Description of the Related Art
In a magneto-optical recording method, a part of a magnetic film in a magneto-optical recording medium is locally heated to a Curie temperature or a compensation temperature or higher by irradiating laser beams thereto. The heated part is then magnetized to the direction of an external magnetic field, thereby forming domains.
A magnetic field modulation recording method is an exemplary magneto-optical recording method for such a magneto-optical recording medium In the magnetic field modulation recording method, the temperature of an overall recording magnetic film is increased by irradiating laser beams having a predetermined intensity thereto and a thermo-magnetical recording is performed onto a certain portion of the heated recording magnetic film by using an external magnetic field having a direction modulated in accordance with a signal to be recorded.
A light power modulation recording method is another exemplary magneto-optical recording method. In the light power modulation recording method, the temperature of a recording magnetic film is increased by irradiating laser beams having an intensity which has been modulated in accordance with a recording signal and simultaneously impressing an external magnetic field having a predetermined intensity, thereby performing a thermo-magnetical recording.
In a conventional magneto-optical recording medium, if the size of a domain becomes equal to or smaller than the diameter of a readout beam spot, the adjacent domains that are located prior to and posterior to the domain from which information is to be read out are also included unintentionally within the readout beam spot (i.e., the detection range). In such a case, the magnitude of a read out signal is reduced because of the interaction from these adjacent domains, the S/N ratio thereof is adversely decreased.
In order to 'solve such a problem, a magneto-optical recording method such as that shown in FIGS. 1A and 1B has been proposed (see Nikkei Electronics, No. 539, Oct. 28, 1991)A Hereinafter, such a magneto-optical recording method will be briefly described.
As shown in the cross-sectional view in FIG. 1B, a magneto-optical recording medium 60 is formed so as to include a readout magnetic film 63, a copy magnetic film 64A, an intermediate film 64, and a recording magnetic film 65, all of these films having been stacked in this order on a substrate (not shown). In FIG. 1B, the arrow X indicates the tangential direction of the magneto-optical recording medium 60 along the tracks thereof, an upward arrow 61 indicates the magnetic fields for recording and readout, and a downward arrow 62 indicates an initialization magnetic field.
On the other hand, FIG. 1A is a plan view showing a part of a track on the magneto-optical recording medium 60. As shown in this figure, when information. is read out, a readout beam spot 67 is formed by an irradiated laser beam along the track. When the laser beam is irradiated onto a rotating magneto-optical recording medium 60, the temperature distribution of the magnetic film structure including the readout magnetic film 63 and the copy magnetic film 64A is not in rotation symmetry with respect to the center of a circle formed by the readout beam spot 67. More specifically, a temperature area 70 which has already been irradiated with the readout beam spot 67 becomes a high temperature area 70, in which the temperature rises to the Curie temperature Tc of the copy magnetic film 64A or higher. A crescent part occupying the left area of the readout beam spot 67 outside of the high temperature area 70 will be called an "intermediate temperature area" 72, while the remaining internal part of the readout beam spot 67 on the right side of the intermediate temperature area 72 will be called a "low temperature area" 71.
Herein, it is assumed that signals (or information) have been recorded beforehand as domains 69 in the recording magnetic film 65 in a thermo-magnetical manner. The copy magnetic film 64A has been strongly exchange-coupled with the readout magnetic film 63. The intermediate film 64 is a film provided for stabilizing a domain wall when the magnetized direction of the readout magnetic film 63 is aligned with that of the recording magnetic film 65.
Hereinafter, the readout operation of the magneto-optical recording medium 60 having such a configuration will be described.
Initially, the magnetized direction of the readout magnetic film 63 is aligned with the direction of the initialization magnetic field 62. During readout, a laser beam for readout is irradiated onto the range from X1 to X2 shown in FIG. 1B onto the rotating magneto-optical recording medium 60 to form the readout beam spot 67. As a result, the temperature is increased and the temperature distribution shown in FIG. 1A (i.e., the respective temperature areas 70, 71, and 72 as described previously) is produced on the magneto-optical recording medium 60. In this case, since the coercive force of the readout magnetic film 63 is decreased because of the rise in temperature, the exchange-coupling with the recording magnetic film 65 becomes dominant in the intermediate temperature area 72 and the magnetized direction of the readout magnetic film 63 is aligned with the magnetized direction of the recording magnetic film 65.
Furthermore, in the high temperature area 70 having a temperature of Tc or higher, the magnetization of the copy magnetic film 64A becomes extinct. As a result, the exchange-coupling between the readout magnetic film 63 and the recording magnetic film 65 is cut off at the area corresponding to the extinct magnetization, and the magnetized direction of the readout magnetic film 63 is aligned in the direction of the readout magnetic field 61, Thus, both the low temperature area 71 inside the readout beam spot 67 and the high temperature area 70 mask their specific domains 69. Consequently, information can be read out as a read out signal from the domain 69X existing only in the intermediate temperature area 72.
In this manner, in accordance with the above-described method, even if a domain 69 is smaller than the readout beam spot 67, information can be read out at a high density without causing any interaction from the adjacent domains 69.
However, the above-described magneto-optical recording medium 60 has a problem in that the initialization magnetic field 62 is required for performing an initialization process for aligning the magnetized direction of the readout magnetic film 63 with the direction of the recording magnetic film 65.
In order to solve such a problem, a magneto-optical recording medium 80 such as that shown in FIGS. 2A and 2B has been proposed (see Japanese Laid-Open Patent Publication No. 5-81717).
The magneto-optical recording medium 80 includes a readout magnetic film 83 and a recording magnetic film 85 which have been formed on a substrate (not shown). in the cross-sectional view in FIG. 2B. In FIG. 2B, the arrow X indicates the tangential direction of the magneto-optical recording medium 80 along the tracks. In the magneto-optical recording medium 80, an in-plane anisotropic magnetic film is used as the readout magnetic film 83, unlike the magneto-optical recording medium 60 shown in FIGS. 1A and 1B.
FIG. 2A is a plan view showing a part of a track on the magneto-optical recording medium 80. As in the magneto-optical recording medium 60 described with reference to FIGS. 1A and 1B, when information is read out, a readout beam spot 87 is formed by irradiating a laser beam onto the range from X1 to X2 shown in FIG. 2B along the track.
When the laser beam. is irradiated onto the rotating magneto-optical recording medium 80, the temperature distribution of the readout magnetic film 83 and the recording magnetic film 85 is not in rotation symmetry with respect to the center of a circle formed by the readout beam spot 87. More specifically, an area which has already been irradiated with the readout beam spot 87 in addition to the left end area of the readout beam spot 87 forms a high temperature area 90. On the other hand, the remaining area of the readout beam spot 87 outside of the high temperature area 90 becomes a low temperature area 91. In this case, a domain 89 is also much smaller than the readout beam spot 87.
Hereinafter, the readout operation of the magneto-optical recording medium 80 having such a configuration will be described.
Herein, it is also assumed that recording signals have been recorded beforehand in each domain 89 of the recording magnetic film 85 In accordance with the thermo-magnetical recording method. The readout magnetic film 83 exhibits an in-plane anisotropy at room temperature, and is turned into a perpendicular magnetic film only in the high temperature area 90 inside the readout beam spot 87. When a readout laser beam is irradiated onto the range from X1 to X2 shown in FIG. 2B, the temperature is increased so that the high temperature area 90 and the low temperature area 91 are formed. In the high temperature area 90, the readout magnetic film 83 is changed into a perpendicular magnetic film and the magnetized direction thereof is aligned with the magnetized direction of the recording magnetic film 85 owing to the exchange-coupling. However, when the temperature is decreased as the magneto-optical recording medium 80 moves to the X direction, the readout magnetic film 83 is changed again into an in-plane anisotropic magnetic film.
In this way, in the magneto-optical recording medium 80, information can be read out from a domain 89 smaller than the readout beam spot 87 without using the initialization magnetic field.
However, though such a magneto-optical recording medium 80 using an in-plane anisotropic magnetic film as the readout magnetic film 83 can advantageously eliminate the initialization magnetic field, the magneto-optical recording medium 80 also has the following disadvantages.
First, the magnetization of the readout magnetic film 83 is attracted to the direction of the recording magnetic film 85 owing to the exchange-coupling between the readout magnetic film 83 and the recording magnetic film 85. As a result, an ideal in-plane anisotropy cannot be maintained and a vertical component of magnetization is produced. Consequently, a copy of the domains 89 is adversely caused in the areas where such a copy is unnecessary, and the resolution may be insufficient at the time of readout.
Second, since the critical temperature at which the readout magnetic film 83 is changed from an in-plane anisotropic magnetic film into a perpendicular magnetic film is constant, a variation in readout power of a readout laser beam varies the areas to which the domains 89 are copied. Consequently, an interaction of waveform of the readout signals is caused among the adjacent domains so that the readout characteristics are degraded.